The invention relates to apparatus for damping the fluctuations (such as vibrations and/or peaks) of torque which is being transmitted between a plurality of rotary masses, for example, in a composite flywheel wherein a first or primary mass or flywheel is rotatable with as well as relative to a second or secondary mass or flywheel, or vice versa.
More particularly, the invention relates to improvements in apparatus which can be utilized with advantage to oppose undesirable stray movements (such as vibratory movements, abrupt accelerations or abrupt decelerations) of one or more parts which receive torque from a crankshaft, a camshaft or another rotary output element of a prime mover (such as the combustion engine of a motor vehicle) and which serve to transmit torque to an input element (such as the pressure plate or the clutch disc of a friction clutch) which, in turn, transmits torque to one or more rotary input elements of one or more driven assemblies (e.g., a manual, automated or automatic change-speed transmission, a differential, a light generator, an air conditioning system, a constituent of a power steering mechanism and/or others).
It is well known to provide the power train of a motor vehicle with a vibration damping system which is called a composite flywheel and employs two coaxial masses or flywheels (including a primary mass which is attached to the output element of the engine and a secondary mass which can transmit torque to a transmission, e.g., by way of a suitable clutch) which are rotatable with each other as well as relative to each other against the resistance of a damping device or other suitable means for yieldably opposing rotation of the primary and secondary masses relative to each other.
It is also known to employ a damping mechanism which includes a planetary transmission (hereinafter called planetary for short) and energy storing means acting in the circumferential direction of the primary and secondary masses. An advantage of a planetary between the primary and secondary masses of a torsional vibration damping apparatus (as used herein, the term "torsional vibration" is intended to embrace all undesirable stray movements which should not be transmitted from the primary to the secondary mass of a composite flywheel or an analogous vibration damping apparatus or system, e.g., in the power train between the engine and the wheels of a motor vehicle, such as might be caused for example by inertia, inaccurate centering and/or for other reasons) is that the planetary can eliminate or weaken the influence of fluctuations of torque resulting from the inertia of rotating masses and/or from the damping of transmitted torque. Otherwise stated, the planetary can exert a beneficial influence upon the behavior of a torsional vibration damping apparatus as a result of appropriate selection of the transmission ratio.
For example, a presently known torsional vibration damping apparatus which is to be effective between two masses or flywheels adapted to rotate about a common axis with as well as relative to each other is designed to enhance the comfort of the occupant(s) of a motor vehicle in that its energy storing means (e.g., a coil spring or a set of coil springs) is designed in such a way that its spring rate or stiffness is reduced at least within the first stage or portion of its characteristic curve. This can be readily achieved if the energy storing means employs a series of coil springs. Furthermore, such energy storing means can be utilized in conjunction with two masses which are rotatable relative to each other and which can effect a change of the moment of inertia as a function of movement; this can result in a reduction (or even elimination) of peaks (if any) of transmitted torque. Thus, it is possible to equip the power train of a motor vehicle with a composite flywheel which is specifically designed and particularly well suited for use in a specific make of a motor vehicle.
The kinematics of a planetary which operates between the primary and secondary masses of a composite flywheel can be selected to exert a beneficial influence upon the composite flywheel as well as upon the entire power train within a wide range of the magnitudes of transmitted torque. For example, if the planetary is designed in such a way that its input side is being acted upon by mutually opposing damping forces in the free mass, it is possible to reduce the peaks of torque acting upon the output element (such as a camshaft or a crankshaft) of an engine. Furthermore, the planetary can reduce undesirable accelerations of RPM at the output side, i.e., at the side which is remote from the prime mover and serves to transmit torque to a transmission, e.g., by way of a friction clutch or any other suitable clutch. This, in turn, prolongs the useful life of various auxiliary aggregates (such as the aforementioned light generator (dynamo), the air conditioning system, the pump of the power steering mechanism and others) which receive motion from the output element of the engine downstream of the flywheel.
To summarize, available torsional vibration damping apparatus of the above outlined character can take advantage of three important expedients or principles, namely friction, velocity-related damping, and acceleration related damping.
In certain presently known torsional vibration damping apparatus, the primary mass is rotatable relative to at least one carrier forming part of a planetary and mounting at least one planet pinion which meshes with a sun gear or sun wheel and an internal gear or wheel. One of the primary and secondary masses of such apparatus is provided with an abutment which acts upon the energy storing means of the apparatus between the primary and secondary masses whereby the energy storing means bears upon an intermediate mass (e.g., a mass including the sun gear, the planet carrier(s) and the internal gear). The parameters of movement of the intermediate mass vary as a function of variations of the extent and direction of angular displacements of the primary and secondary masses relative to each other.
FIGS. 3 and 4 of German patent No. 31 39 658 C2 illustrate a torsional vibration damping apparatus wherein the primary mass is provided with a friction lining and is fixedly secured to a sun gear on a hub of a secondary mass. The planetary which includes the sun gear further comprises a planet carrier which is secured to the hub and has limited freedom of movement relative to the friction lining. The planet pinions which are mounted on the carrier mesh with the sun gear as well as with an internal gear on one or more covering panels for the hub. The internal gear can turn relative to the hub. The panels have windows for discrete springs of the energy storing means.
In the just described conventional torsional vibration damping apparatus, torque which is being transmitted to the primary mass is transmitted to the sun gear by way of the friction lining. In the event of fluctuations of transmitted torque, the planet pinions rotate relative to the sun gear and relative to the internal gear to thus effect a change in the angular position of the aforementined panels relative to the hub. This entails a stressing of the springs, i.e., the energy storing means is caused to store energy.
By properly selecting the ratio of the planetary, the extent of deformation of the springs in the windows of the panels can be selected in such a way that it is more satisfactory than in a torsional vibration damping apparatus which does not employ a planetary, i.e., wherein the springs of the energy storing means are stressed to an extent which is directly proportional to the extent of angular movement of the primary and secondary masses relative to each other. It has been found that the stressing of springs, which form part of the energy storing means, by way of a planetary is much more effective to reduce or to eliminate the influence of undesirable fluctuations of transmitted torque. However, the just described torsional vibration damping apparatus also exhibit a serious drawback, namely the mass moment of inertia at the output side is small which affects the ability of the patented apparatus to counteract or absorb pronounced fluctuations of transmitted torque.
A torsional vibration damping apparatus which can counteract more pronounced fluctuations of torque and is installed between the primary and secondary masses of such apparatus (e.g., a twin-mass flywheel) is disclosed, for example, in German patent No. 36 30 398 C2. This rather rudimentary apparatus merely employs a set of springs which are stressed to a greater or lesser extent, depending upon the magnitude of the angle of rotation of the primary and secondary masses relative to each other. A drawback of such apparatus is that their ability to absorb or counteract fluctuations of torque is rather limited though they are capable of absorbing some of the fluctuations regardless of their magnitude.
A comparison of torsional vibration damping apparatus which merely employ a set of springs with those which employ energy storing means in conjunction with planetaries indicates that those using planetaries exhibit at least some important advantages including the following:
If the primary mass receives torque so that it begins to rotate relative to the secondary mass (or vice versa), the primary mass transmits a first portion of the torque to the secondary mass whereas the remaining second portion of such torque is transmitted to the aforementioned intermediate mass (which can include the sun gear, the planet carrier and the internal gear of the planetary). The ratio of the first and second portions of the torque being transmitted by the primary mass (as concerns the magnitudes and the directions of action of the first and second portions of the torque) is dependent upon the construction of the planetary and the manner in which the planetary is operatively connected with the primary and/or with the secondary mass. It is possible to design and install the planetary in such a way that the magnitude of the first and/or the second portion of the engine torque being transmitted by an apparatus employing a planetary exceeds the torque being transmitted to the primary mass. However, by mounting the springs of the energy storing means in two sets one of which damps the fluctuations (if any) of the first portion of transmitted torque and the other of which damps the fluctuations of the second portion of transmitted torque, it is possible to ensure that the torque being transmitted by the secondary mass a least approximates the torque being transmitted to the primary mass. The two sets of springs of the energy storing means do not undergo any pronounced deformation but can effect a desirable "smoothing" of the respective portions of the torque being transmitted by the primary mass, i.e., the fluctuations of the torque being transmitted by the secondary mass are or can be much less pronounced than those of the torque being transmitted to and by the primary mass. Moreover, the inertia of the secondary and intermediate masses does not greatly affect the quality of the damping action provided that the magnitudes of the first and second portions of the torque being transmitted by the primary mass are substantial. On the other hand, if the magnitudes of the two portions of the torque being transmitted from the primary mass (a) directly to the secondary mass and (b) to the intermediate mass are relatively small, the difference between the RPM of the secondary mass and the RPM of the intermediate mass is very pronounced with the result that the springs undergo a substantial deformation and the inertia of the secondary and intermediate masses appears to be much greater.
The above enumerated experiences with conventional torsional vibration damping apparatus employing a planetary would indicate that the planetary and its connection with at least one of the primary and secondary masses should be selected with a view to ensure a pronounced damping of the fluctuations of transmitted torque prior to the transmission of the thus influenced torque to the part or parts being driven by the secondary mass. This holds true irrespective of which of the masses (other than the intermediate mass) acts as a primary mass or a secondary mass. It is to be borne in mind that the ratio of a planetary changes when a vehicle is coasting, i.e., if the normally secondary mass serves to transmit torque to the normally primary mass because, in lieu of transmitting motion from the sun gear, to the internal gear the planet pinions then transmit motion from the internal gear to the sun gear (or the other way around).